An 'undefined' page is any page that has been linked to from within Confluence, but that does not yet exist.

Undefined Page Linked From
Migrating projects from Teamwork Server Page: Resaving server projects with latest version in Teamwork Server (MagicDraw 19.0 LTR SP2)
Specification window Page: Locking a model for editing in Teamwork Server (MagicDraw 19.0 LTR SP2)